1. Field of the Invention
The present invention relates generally to fuel dispensing systems such as the type commonly installed at gasoline service stations and, more particularly, to a removable plug for sealing a testing port of a distribution head used in fuel distribution systems. Of course, the removable plug has general application for sealing a port and is, therefore, not limited to the fuel dispensing environment.
2. Description of the Related Art
Gasoline service stations normally have underground storage tanks from which the fuel is pumped to dispensing units. A typical installation makes use of a submersible electric motor driven pump in the storage tank which operates to pump gasoline or another fuel to a distribution head located above the tank. From the distribution head, the fuel is supplied to the dispensers. The flow path for the fuel includes a vertical column pipe which extends from the pump to the distribution head.
Many city codes require that pipelines from the distribution head to dispensing units be pressure tested for leaks. In order to conduct periodic testing and maintenance it is necessary to access various devices within the distribution head. This is done by removing a plug from a testing port and inserting a pressure gauge within the port. It is essential that the port is securely sealed because of fluid pressure that acts upon the plug during normal use. Typically, the plug is engaged in the port by either machine threads or the type of threads referred to by the American National Standards Institute (ANSI) as American National Standard Taper Pipe Threads (NPT).
Known fuel distribution systems which utilize such plugs include, U.S. Pat. No. 3,172,567, U.S. Pat. No. 3,172,572 and U.S. Pat. No. 3,197,085, issued Mar. 9, 1965, Mar. 9, 1965 and Jul. 27, 1965, respectfully. These systems describe header assemblies with a port and plug having machine type threads which are sealed with an o-ring. The plug tightens when it bottoms out at the top of the threads and the o-ring seals the joint; an interference fit is not formed between the threads on the plug and in the port. Any device which is threaded into the port must have machine type threads, and must rely on an o-ring to form a seal.
Other known fuel pumping systems, as shown in FIG. 7, utilize a plug 2 and port 4 having NPT threads 6. Typically, a sealant 8 is applied to the threads 6 of the plug 2 to create an air tight seal. The plug 2 is securely held in place by an interference fit which is formed between the threads 6 on the plug interfering with the threads 6 on the plug 2, as opposed to machine threads which rely on a positive stop formed by a bolt head, as mentioned above. The interference fit is caused by the thread's 6 tapered diameters which increasingly resist each other during engagement. However, the combination of the sealant 8 and thread interference result in a very tight fit which makes the plug 2 difficult to remove from the port 4. Furthermore, the port is often located within a manhole having limited clearance; thus, making the removal process of the plug even more difficult.